In the efforts for optimizing and improving operation in various high-speed microcontroller-based devices, such as various instrumentation and measurement equipment and the like, significant attention has been given to the further improvement of the high-speed amplifiers utilized. Examples of high-speed amplifiers commonly utilized include inverting amplifiers, gain amplifiers, and transimpedance amplifiers. Transimpedance amplifiers can be used to convert low-level photodiode currents to usable voltage signals. Such transimpedance amplifiers are more commonly implemented within low current and leakage current measurement applications, as well as other low-level sensor current applications.
With reference to FIG. 1, a prior art composite amplifier circuit 100 as may be implemented within a transimpedance amplifier application is illustrated. Composite amplifier circuit 100 comprises a high-speed operational amplifier 102 configured for providing an output voltage at output terminal VOUT. Composite amplifier circuit 100 can also comprise an integrator circuit including an auto-zero amplifier 104, a resistor R1 and a capacitor C1. Auto-zero amplifier 104 is configured for offset correction, e.g., correction of input voltage offset, of high-speed amplifier 102. Auto-zero amplifier 104 is coupled to the input terminals of high-speed amplifier 102, e.g., an inverting input terminal coupled through resistor R1 to an inverting input terminal of high-speed amplifier 102, and an output terminal coupled to the non-inverting input terminal of high-speed amplifier 102. Capacitor C1 is coupled between the inverting input terminal of auto-zero amplifier 104 and the non-inverting input terminal of high-speed amplifier 102.
With additional reference to FIG. 2, a prior art amplifier circuit 200 that may include a composite amplifier 102 configured within a transimpedance application is illustrated. For example, amplifier circuit 200 comprises a composite amplifier 202 having a high-speed amplifier 102 and an auto-zero amplifier 104 and configured with external feedback elements, such as resistor R0 and capacitor C0, coupled between an inverting input terminal and an output terminal of composite amplifier 202. In the transimpedance application, composite amplifier 202 can be coupled to a photo-diode device, represented by current source ID and parasitic capacitances CPAR, through an inverting input terminal.
During operation of transimpedance amplifier circuit 200, it is desirable for the inverting input terminal of composite amplifier 202 to operate at common mode voltage, e.g., zero volts. However, when transimpedance amplifier circuit 200 is operating under an overload condition, with an output voltage close to a supply voltage, current flow within feedback resistor R0 is limited. The input current flowing to transimpedance amplifier circuit 200 forces the node at the inverting input terminal of composite amplifier 202 to be pulled down below common mode voltage. This condition causes a charge to be added across capacitor C1 of composite amplifier 202, and the output of transimpedance amplifier circuit 200 becomes saturated.
Once transimpedance amplifier circuit 200 transitions from the overload condition, the charge on capacitor C1 of composite amplifier 202 slowly discharges due to the slow operation of auto-zero amplifier 104. As a result, a slow tail voltage is realized at output terminal VOUT. This slow tail voltage causes an increase in the overload recovery time, thus reducing the dynamic range of transimpedance amplifier circuit 200.
One technique used to improve the overload recovery time is to limit the amount of current flowing within feedback resistor R0 during overload condition through the use of a zener-based device configured in parallel with feedback resistor R0 to clamp the voltage drop across feedback resistor R0. Unfortunately, this zener-based device clamp configuration causes a “soft knee” affect in the output voltage of composite amplifier 202, generates errors within the normal operating region, and limits the transimpedance output signal dynamic range.